An asynchronous remote copy system for copying data, which has been written to a primary volume of a primary storage system, to a secondary volume of a remotely located secondary storage system in an asynchronous manner with respect to the write to the primary volume is known. According to an asynchronous remote copy system, a plurality of data written to a primary volume is written to a secondary volume in the order in which the data was written to the primary volume.
In an asynchronous remote copy, there is a difference between the time that data was stored in the primary storage system and the time that the data was stored in the secondary storage system. Hereinafter, this difference will be referred to as the “delay duration”. The delay duration may deteriorate, which means that remote copy reliability has declined. PTL 1 discloses a technology for monitoring the delay duration.